French authorities found three kilos of explosives, several guns and an Islamic State jihadist flag after arresting two men Tuesday suspected of plotting an attack, just days before France votes in presidential polls, a prosecutor said.

Paris prosecutor Francois Molins told reporters it was not clear when or where the suspects planned to strike, but he said their attack had been "imminent".

The items were discovered during searches in Marseille after the suspects, both "radicalized" Frenchmen aged 23 and 29, were taken into custody.

More than 230 people have been killed in terror attacks in France since January 2015.

Candidates have been heavily guarded during the election campaign, but so far there have been few security scares.

"Everything will be done to ensure security" for the election, Fekl said.

The race was narrowing ahead of Sunday's vote, with the pack closing behind front-runners Emmanuel Macron and Marine Le Pen, while a quarter of voters remained undecided.

For weeks, centrist former banker Macron and National Front (FN) leader Le Pen have been out in front but opinion polls now show there is a very real chance that any of the four leading candidates could reach the second-round runoff on May 7.

Scandal-plagued conservative Francois Fillon and far-left firebrand Jean-Luc Melenchon have closed the gap substantially in the last two weeks.

"We have never seen a four-way contest like this in the first round of a presidential election," Frederic Dabi of the Ifop polling institute told AFP.

"There has been a real tightening of the race with four candidates between 19 percent and 23 percent," he added.

Macron and Le Pen are tied on 22-23 percent, with Fillon improving to around 21 percent and Melenchon surging as high as 20 percent in some polls.

EU fears

With Le Pen expected to reach the second round, polls continue to indicate that whoever faces her will win, although after Brexit and Donald Trump's US election win, no one is taking anything for granted.

Melenchon has made the most remarkable breakthrough in recent weeks, surging as high as 20 percent in some polls with a far-left program that involves huge public spending and a pledge to re-negotiate all European Union treaties.

Le Pen wants to pull France out of the eurozone and also foresees a mass re-negotiation of EU treaties, sparking fears in Brussels that a victory for the far-right coming hot on the heels of Brexit could be fatal for the European bloc.

In contrast to Le Pen, Macron on Sunday told 20,000 people in Paris that France's future lay firmly in Europe, albeit one that suited French interests.

"We need Europe, so we will remake it," Macron told the crowd. "I will be the president of the awakening of our European ambitions."

In a reference to Le Pen, Macron said voters had the choice of "hope and courage over resignation".

Abstention fears

Polls also show that more than a quarter of voters have yet to decide who they will support on Sunday.

Analysts say there is a growing danger of a high abstention rate affecting the result, possibly tipping the scales in favor of Fillon.

"The behavior of people who are currently planning not to vote is a major factor. If those on the right (who were going to abstain) decide to vote for Fillon, it changes things," Ifop's Dabi said.

"It is one of the unknowns of a vote that has many unknowns."

Fillon, a 63-year-old former prime minister, has been charged over allegations he gave his wife Penelope a fictional job as his parliamentary assistant for which she earned nearly 680,000 euros ($725,000) in public money.

Having led the polls at the turn of the year, the scandal hit his support hard.

But voters appear to be warming to his message that he is a safe pair of hands in an election of largely untested candidates.

Fillon said Sunday he was confident he would upend the polls and reach the second round.